revert: channel unions (#3918 69d69f2)

[SpaceEEC]

Please describe the changes this PR makes and why it should be merged:

This PR reverts #3918 / 69d69f2.
(Aside from StoreChannel#type and GuildChannel#setParent, as those are unrelated to this)

Taking the following example here:

guild.channels.cache.get(someID).setName('foo').then(channel => {
  console.log(`New name: ${channel.name}`);
});

Results in an error like this:

This expression is not callable.
  Each member of the union type
(<TResult1 = CategoryChannel, TResult2 = never>(onfulfilled?:
(value: CategoryChannel) =>TResult1 | PromiseLike<TResult1>, onrejected?:
(reason: any) => TResult2 | PromiseLike<...>) => Promise<...>)
| (<TResult1 = NewsChannel, TResult2 = never>(onfulfilled?: (value: NewsChannel) =>
TResult1 | PromiseLike<...>, onre...
has signatures, but none of those signatures are compatible with each other.

However using async / await seems to work fine:

const channel = await guild.channels.cache.get(someID).setName('foo');
console.log(`New name: ${channel.name}`);

Considering that it works when using async / await, this feels like an issue with TS rather than our typings.

A d.js-free example of this error:

function a() {
	return Promise.resolve('a');
}

function b() {
	return Promise.resolve([1]);
}

function c() {
	if (Math.random() > 0.5) return a();
	return b();
}

c().then(thing => {
	console.log(`thing has a length of ${thing.length}`);
});

However there probably is nothing we can really do about this for now, except reverting.

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.